datetimeFormat = flag.String("datetime-format", "%Y-%m-%d_%H", "strftime compatible format for <DATETIME> in filename format")
filenameFormat = flag.String("filename-format", "<TOPIC>.<HOST><REV>.<DATETIME>.log", "output filename format (<TOPIC>, <HOST>, <PID>, <DATETIME>, <REV> are replaced. <REV> is increased when file already exists)")

gzipLevel      = flag.Int("gzip-level", 6, "gzip compression level (1-9, 1=BestSpeed, 9=BestCompression)")
gzipEnabled    = flag.Bool("gzip", false, "gzip output files.")

rotateSize     = flag.Int64("rotate-size", 0, "rotate the file when it grows bigger than `rotate-size` bytes")
rotateInterval = flag.Duration("rotate-interval", 0*time.Second, "rotate the file every duration")

skipEmptyFiles = flag.Bool("skip-empty-files", false, "Skip writing empty files")

func NewFileLogger(gzipEnabled bool, compressionLevel int, filenameFormat, topic string) (*FileLogger, error) {
	// TODO: remove, deprecated, for compat <GZIPREV>
	// 默认格式为："<TOPIC>.<HOST><REV>.<DATETIME>.log"
	// strings.Replace：If n < 0, there is no limit on the number of replacements.
	filenameFormat = strings.Replace(filenameFormat, "<GZIPREV>", "<REV>", -1) // filenameFormat中的<GZIPREV>替换为<REV>，
	if gzipEnabled || *rotateSize > 0 || *rotateInterval > 0 {

		// 指定压缩、文件滚动后，文件名格式中必须存在<REV>字段
		// strings.Index:如果字符串中没有出现指定字段，函数返回-1
		if strings.Index(filenameFormat, "<REV>") == -1 {
			return nil, errors.New("missing <REV> in --filename-format when gzip or rotation enabled")
		}

	} else { // remove <REV> as we don't need it
		filenameFormat = strings.Replace(filenameFormat, "<REV>", "", -1)
	}

	hostname, err := os.Hostname()	// 获取操作系统的主机名称
	if err != nil {
		return nil, err
	}
	shortHostname := strings.Split(hostname, ".")[0]
	identifier := shortHostname
	if len(*hostIdentifier) != 0 { // 根据opt需要，决定HOSTNAME的长度
		identifier = strings.Replace(*hostIdentifier, "<SHORT_HOST>", shortHostname, -1)
		identifier = strings.Replace(identifier, "<HOSTNAME>", hostname, -1)
	}
	filenameFormat = strings.Replace(filenameFormat, "<TOPIC>", topic, -1)
	filenameFormat = strings.Replace(filenameFormat, "<HOST>", identifier, -1)
	// 通过区分PID，实现支持多终端运行时，对不同文件的写操作。
	filenameFormat = strings.Replace(filenameFormat, "<PID>", fmt.Sprintf("%d", os.Getpid()), -1)

	// 判断是否需要加压缩后缀.gz
	if gzipEnabled && !strings.HasSuffix(filenameFormat, ".gz") {
		filenameFormat = filenameFormat + ".gz"
	}

	f := &FileLogger{
		logChan:          make(chan *nsq.Message, 1),
		compressionLevel: compressionLevel,
		filenameFormat:   filenameFormat,
		gzipEnabled:      gzipEnabled,
		ExitChan:         make(chan int),
		termChan:         make(chan bool),
		hupChan:          make(chan bool),
	}
	return f, nil
}

type FileLogger struct {
	out              *os.File
	writer           io.Writer
	gzipWriter       *gzip.Writer
	logChan          chan *nsq.Message
	compressionLevel int
	gzipEnabled      bool
	filenameFormat   string

	ExitChan chan int
	termChan chan bool
	hupChan  chan bool

	// for rotation
	lastFilename string
	lastOpenTime time.Time
	filesize     int64
	rev          uint
}

func (f *FileLogger) calculateCurrentFilename() string {
	t := time.Now()
	datetime := strftime(*datetimeFormat, t) //根据opt的时间格式要求，生成时间戳文件名
	return strings.Replace(f.filenameFormat, "<DATETIME>", datetime, -1)
}

func hasArg(s string) bool {
	argExist := false
	flag.Visit(func(f *flag.Flag) {
		if f.Name == s {
			argExist = true
		}
	})

	return argExist
}

flag.Visit(func(f *flag.Flag) {
	if f.Name == s {
		argExist = true
	}
})

// Visit visits the flags in lexicographical order, calling fn for each.
// It visits only those flags that have been set.
func (f *FlagSet) Visit(fn func(*Flag)) {
	for _, flag := range sortFlags(f.actual) {
		fn(flag)
	}
}